1. Technical Field
The disclosure relates to thermally curable solder resist composition.
2. Description of the Related Art
Flexible printed circuit boards are flexible, and satisfy requirements of being light weight, thin, and small for electronic products and are widely applied to various industries such as the aircraft industry, the mechanical industry, the automobile industry and the electronics industry. Because copper wires on the flexible printed circuit boards are easily oxidized by environmental influences, a protective film with functions similar to a solder mask is required covering the copper wires to prevent oxidization of the copper wires. A solder mask is an ink consisting of epoxy resin, which is brittle and not flexible enough for flexible printed circuit boards after being formed as a film on flexible printed circuit boards. Thus, solder masks are not suitable for use in flexible printed circuit boards.
The conventional material of protective films used in flexible printed circuit boards is a polyimide (PI) or an acrylic protective film containing one layer of epoxy resin adhesive. While the above mentioned materials can be applied to copper clad laminates with no adhesives, the structure of the materials thereof would be asymmetric and thus flexibility of flexible printed circuit board using the above mentioned materials would be insufficient. Additionally, heat resistance of epoxy resin adhesive layers is poor and shrinkage thereof is large, thus negatively effecting heat resistance and dimensional stability of flexible printed circuit boards made of the above mentioned materials. Moreover, the above mentioned materials are applied to chip on flex (COF) products, wherein flexible printed circuit boards made thereof are required to be good bending resistance. However, because conventional PI protective films containing epoxy resin adhesive layers are too rigid, fixing problems occur when bending the flexible printed circuit boards made of the above mentioned materials. Thus, conventional material of protective films used in flexible printed circuit boards can not be used in chip on flex (COF) products.
A liquid protective film material used in flexible printed circuit boards is made by an epoxy resin system, which requires adding rubber as a soft agent to improve flexibility thereof. However, heat resistance of rubber is poor and compatibility of rubber with epoxy resin is insufficient. If too much rubber is added, heat resistance of the protective film would be poor and problems would occur during fabrication soldering processes at high temperatures. However, if too little rubber is added, the flexibility of the protective film would be poor and the protective film would easily crack.
U.S. Pat. No. 6,818,702 and US Publication No. 2007/0088134 disclose a modified epoxy resin with flexibility characteristics. Polybutadiene with end groups containing —COOH or an —OH functional group is used to react with epoxy resin. Although the flexibility of the above mentioned materials is improved, the synthesis steps of the above mentioned materials are complex and heat resistance and the adhesion thereof are poor. Thus, the modified epoxy resin with flexibility characteristics easily delaminated from substrates during subsequent processes.
Meanwhile, Japan Patent No. JP2006124681 and JP2007246648 and US Publication No. 2007/0293636 disclose utilizing urethane to modify epoxy resin. Polycarbonate and diisocyanate having —OH functional groups are reacted with dimethylopropionic acid to form urethane having a —COOH functional group, and then the urethane is mixed with epoxy resin. Although, the above mentioned materials have good flexibility, the ratio of urethane is too high, such that heat resistance and the chemical resistance thereof are poor. While an acrylic rubber may be utilized to be added to the epoxy resin to improve flexibility, the heat resistance of acrylic rubber is lower than epoxy resin, thus negatively effecting heat resistance thereof.
Therefore, a solder resist composition with sufficient flexibility, sufficient heat resistance and other properties to satisfy requirements of protective films of flexible printed circuit boards is desired.